In a preliminary design available information is limited, so it makes attractive to rely on the design cases of existing ships to design new ships. In this paper a prototype of the case-based conceptual design system of a ship is developed to support systematically the design process. This system not only generates design candidates through a case indexing, but also determines the priorities of the candidates by using nearest neighbor matching algorithm. The final solution is presented through adaptation process. The validation of the system was examined and verified by applying to conceptual ship design stage.